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Nuclear weapon

Anuclear
weaponis an
explosive device that derives its destructive force fromnuclear reactions, eitherfission(fission bomb) or a combination of fission andfusion(thermonuclear weapon).
Both reactions release vast quantities of energy from relatively small amounts
of matter. Thefirst testof afission ("atomic")
bombreleased
the same amount of energy as approximately 20,000tons of TNT(84 TJ).
The firstthermonuclear
("hydrogen") bombtestreleased
the same amount of energy as approximately 10 million tons of TNT
(42 PJ).

A thermonuclear weapon weighing little more than 2,400 pounds
(1,100 kg) can produce an explosive force comparable to the detonation of
more than 1.2 million tons of TNT (5.0 PJ).A nuclear device no larger than traditional
bombs can devastate an entire city by blast, fire, andradiation. Nuclear weapons are consideredweapons of mass
destruction, andtheir use and controlhave been a major focus ofinternational
relationspolicy
since their debut.

Nuclear weapons have been used twice innuclear warfare, both times by theUnited StatesagainstJapannear the end ofWorld War II. On August 6, 1945, theU.S. Army Air Forcesdetonated auraniumgun-typefission bombnicknamed "Little Boy" over the
Japanese city ofHiroshima; three days later, on August 9, the
U.S. Army Air Forces detonated aplutoniumimplosion-type fission bomb codenamed
"Fat Man" over the Japanese city ofNagasaki. The bombings resulted in the deaths
of approximately 200,000civiliansandmilitary personnelfrom
acute injuries sustained from the explosions.The ethics of the bombings and their role
inJapan's surrenderremain the subject ofscholarly and popular debate.

Since theatomic
bombings of Hiroshima and Nagasaki, nuclear weapons have been
detonated on over two thousand occasions for the purposes oftestingand demonstration. Onlya few
nationspossess
such weapons or are suspected of seeking them. The only countries known to have
detonated nuclear weapons—and acknowledge possessing them—are (chronologically
by date of first test) theUnited
States, theSoviet Union(succeeded as a nuclear power byRussia),
theUnited
Kingdom,France,
thePeople's Republic of China,India,Pakistan,
andNorth
Korea.Israelis also believed to possess nuclear
weapons, though in apolicy of
deliberate ambiguity, it does not acknowledge having them.Germany,Italy,Turkey,Belgiumand theNetherlandsarenuclear weapons sharingstates.South Africais the only country to haveindependently
developedand
thenrenounced
and dismantledits
nuclear weapons.

Thenuclear
non-proliferation treatyaimed
to reduce the spread of nuclear weapons, but its effectiveness has been
questioned, and political tensions remained high in the 1970s and 1980s. As of
2016, 16,000 nuclear weapons are stored at sites in 14 countries and many are
ready for immediate use.Modernisation
of weapons continues to occur.

Types

There are two basic types
of nuclear weapons: those that derive the majority of their energy from nuclear
fission reactions alone, and those that use fission reactions to beginnuclear fusionreactions
that produce a large amount of the total energy output.

Fission weapons

All existing nuclear weapons derive some of their explosive
energy from nuclear fission reactions. Weapons whose explosive output is
exclusively from fission reactions are commonly referred to asatomic bombsoratom
bombs(abbreviated asA-bombs). This has long been
noted as something of amisnomer,
as their energy comes from the nucleus of the atom, just as it does with fusion
weapons.

In fission weapons, a mass offissile material(enriched uraniumorplutonium)
is assembled into asupercritical
mass—the amount of material needed to start anexponentially growingnuclear chain reaction—either by shooting one
piece of sub-critical material into another (the "gun" method) or by
compressing usingexplosive lensesa sub-critical sphere of material
usingchemical explosivesto many times its original density
(the "implosion" method). The latter approach is considered more
sophisticated than the former and only the latter approach can be used if the
fissile material is plutonium.

A major
challenge in all nuclear weapon designs is to ensure that a significant
fraction of the fuel is consumed before the weapon destroys itself. The amount
of energy released by fission bombs can range from the equivalent of just under
a ton to upwards of 500,000 tons (500kilotons)
ofTNT(4.2 to 2.1×108 GJ).

All fission reactions necessarily generatefission products, the radioactive remains
of the atomic nuclei split by the fission reactions. Many fission products are
either highly radioactive (but short-lived) or moderately radioactive (but
long-lived), and as such are a serious form ofradioactive contaminationif not fully contained. Fission
products are the principal radioactive component ofnuclear fallout.

The most commonly used fissile materials for nuclear weapons
applications have beenuranium-235andplutonium-239.
Less commonly used has beenuranium-233.Neptunium-237and some isotopes ofamericiummay be usable for nuclear explosives
as well, but it is not clear that this has ever been implemented, and even
their plausible use in nuclear weapons is a matter of scientific dispute.

Fusion weapons

The other basic type of nuclear weapon
produces a large proportion of its energy in nuclear fusion reactions. Such fusion
weapons are generally referred to asthermonuclear weaponsor more colloquially ashydrogen bombs(abbreviated asH-bombs), as they rely on
fusion reactions between isotopes ofhydrogen(deuteriumandtritium). All such weapons derive a significant portion, and
sometimes a majority, of their energy from fission. This is because a fission
reaction is required as a "trigger" for the fusion reactions, and the
fusion reactions can themselves trigger additional fission reactions.

Only six countries—United States,
Russia, United Kingdom, People's Republic of China, France and India—have
conducted thermonuclear weapon tests. (Whether India has detonated a
"true", multi-stagedthermonuclear weaponis controversial.)North Koreaclaims to have tested a fusion weapon as
of January 2016, though this claim is disputed.Thermonuclear weapons
are considered much more difficult to successfully design and execute than
primitive fission weapons. Almost all of the nuclear weapons deployed today use
the thermonuclear design because it is more efficient.

Thermonuclear bombs work by using the
energy of a fission bomb to compress and heat fusion fuel. In theTeller-Ulam design, which
accounts for all multi-megaton yield hydrogen bombs, this is accomplished by
placing a fission bomb and fusion fuel (tritium,deuterium, orlithium deuteride) in proximity within a special,
radiation-reflecting container. When the fission bomb is detonated,gamma raysandX-raysemitted
first compress the fusion fuel, then heat it to thermonuclear temperatures. The
ensuing fusion reaction creates enormous numbers of high-speedneutrons, which can then induce fission in materials not
normally prone to it, such asdepleted uranium. Each of these components is known as a
"stage", with the fission bomb as the "primary" and the
fusion capsule as the "secondary". In large, megaton-range hydrogen
bombs, about half of the yield comes from the final fissioning of depleted
uranium.

Virtually all thermonuclear weapons
deployed today use the "two-stage" design described above, but it is
possible to add additional fusion stages—each stage igniting a larger amount of
fusion fuel in the next stage. This technique can be used to construct
thermonuclear weapons of arbitrarily large yield, in contrast to fission bombs,
which are limited in their explosive force. The largest nuclear weapon ever
detonated, theTsar Bombaof the USSR, which released an energy
equivalent of over 50 megatons of TNT (210 PJ), was a three-stage weapon.
Most thermonuclear weapons are considerably smaller than this, due to practical
constraints from missile warhead space and weight requirements.

Edward Teller, often referred to as the "father of the
hydrogen bomb"

Fusion reactions do not create fission
products, and thus contribute far less to the creation of nuclear fallout than
fission reactions, but because all thermonuclear weapons contain at least one fission
stage, and many high-yield thermonuclear devices have a final fission stage,
thermonuclear weapons can generate at least as much nuclear fallout as
fission-only weapons.

Other types

There are other types of nuclear weapons as well. For example, aboosted fission
weaponis a
fission bomb that increases its explosive yield through a small amount of
fusion reactions, but it is not a fusion bomb. In the boosted bomb, the
neutrons produced by the fusion reactions serve primarily to increase the
efficiency of the fission bomb. There are two types of boosted fission bomb:
internally boosted, in which a deuterium-tritium mixture is injected into the
bomb core, and externally boosted, in which concentric shells of
lithium-deuteride and depleted uranium are layered on the outside of the
fission bomb core.

Some weapons are designed for special purposes; aneutron bombis a thermonuclear weapon that yields a
relatively small explosion but a relatively large amount of neutronradiation; such a device could theoretically
be used to cause massive casualties while leaving infrastructure mostly intact
and creating a minimal amount of fallout. The detonation of any nuclear weapon
is accompanied by a blast ofneutron radiation. Surrounding a nuclear
weapon with suitable materials (such ascobaltorgold)
creates a weapon known as asalted bomb. This device can produce
exceptionally large quantities of long-livedradioactive
contamination. It has been conjectured that such a device could
serve as a "doomsday weapon" because such a large quantity of
radioactivities with half-lives of decades, lifted into the stratosphere where
wind currents would distribute it around the globe, would make all life on the
planet extinct.

In connection with theStrategic
Defense Initiative, research into theNuclear pumped laserwas conducted under the Dod programProject Excaliburbut this did not result in a working
weapon. The concept involves the tapping of the energy of an exploding nuclear
bomb to power a single-shot laser which is directed at a distant target.

During theStarfish Primehigh-altitude nuclear test in 1962, an
unexpected effect was produced which is called aNuclear
electromagnetic pulse. This is an intense flash of electromagnetic
energy produced by a rain of high energy electrons which in turn are produced
by a nuclear bomb's gamma rays. This flash of energy can permanently destroy or
disrupt electronic equipment if insufficiently shielded. It has been proposed
to use this effect to disable an enemy's military and civilian infrastructure
as an adjunct to other nuclear or conventional military operations against that
enemy. Because the effect is produced by very high altitude nuclear
detonations, it can produce damage to electronics over a very wide, even
continental, geographical area.

Research has been done into the possibility ofpure fusion bombs:
nuclear weapons that consist of fusion reactions without requiring a fission
bomb to initiate them. Such a device might provide a simpler path to
thermonuclear weapons than one that required development of fission weapons
first, and pure fusion weapons would create significantly less nuclear fallout
than other thermonuclear weapons, because they would not disperse fission
products. In 1998, theUnited
States Department of Energydivulged
that the United States had, "...made a substantial investment" in the
past to develop pure fusion weapons, but that, "The U.S. does not have and
is not developing a pure fusion weapon", and that, "No credible
design for a pure fusion weapon resulted from the DOE investment".

Antimatter, which
consists ofparticlesresembling ordinarymatterparticles in most of their properties but
having oppositeelectric charge, has been considered as a
trigger mechanism for nuclear weapons.A major
obstacle is the difficulty of producing antimatter in large enough quantities,
and there is no evidence that it is feasible beyond the military domain.However, the U.S. Air Force funded studies
of the physics of antimatter in theCold War, and began considering its possible
use in weapons, not just as a trigger, but as the explosive itself.A fourth generation nuclear weapon design
is related to, and relies upon, the same principle asAntimatter-catalyzed
nuclear pulse propulsion.

Most variation innuclear weapon designis for the purpose of achievingdifferent yields for different situations, and
in manipulating design elements to attempt to minimize weapon size.